Laser systems preferentially generate a wave front of a predefined shape such that resulting laser beam has a desired beam quality. In some instances, optical aberrations may be introduced into the wave front by the optical elements of a laser system including optical aberrations introduced by the laser medium, a laser amplifier or other optical components. The optical aberrations may be introduced by the structure and material composition of the optical elements and/or by thermal gradients in the optical elements that create optical path differences. For example, an optical element may be heated uniformly, but may have some portions that are heated to a greater degree than other portions by the laser, thereby creating a thermal gradient. As a result of the thermal gradient across the optical element, the optical element will introduce optical aberrations into the laser beam due to optical path differences occasioned by the thermal gradient. The optical aberrations will, in turn, reduce the efficiency and effectiveness of the laser system by introducing wave front errors and reducing the beam quality generated by the laser system.
In an effort to cancel or offset the optical aberrations, various complex optical systems have been developed. For example, heat transfer and/or cooling systems have been employed in conjunction with laser systems in an effort to reduce thermal gradients and, correspondingly, to reduce the resulting optical aberrations. For example, radially symmetric gain rods may be radially cooled in an effort to reduce thermal gradients. Additionally, fixed corrector plates or complex adaptive optical systems have been utilized in an effort to cancel the optical aberrations introduced by thermal gradients. Although the foregoing techniques may reduce the optical gradients, the resulting laser systems are more complex and, as a result, are generally more expensive.
One type of optical element that may experience a thermal gradient is a photo thermal refractive (PTR) glass optical element. A PTR glass optical element may be utilized, for example, to combine spectral beams in a laser system. The laser beam(s) propagating through a PTR optical element generally increases the temperature of the PTR optical element. As the temperature of the PTR glass optical element increases, the refractive index of the PTR glass optical element changes which, in turn, changes the wavelength of light that is output by the PTR glass optical element. For example, a PTR optical element may operate as a grating such that the increased temperature of the PTR optical element changes the refractive index of the PTR optical element which, in turn, changes the wavelength of the light emitted by the grating which may adversely effect the efficiency of the grating.
In an effort to reduce the temperature increase experienced by a PTR optical element, a PTR optical element may be heated or cooled from its outer edge with a thermal control system. This technique may introduce temperature gradients throughout the volume of the PTR optical element which may, in turn, cause different portions of the PTR optical element to perform differently than other portions of the PTR optical element based upon the thermal gradient, thereby also adversely impacting the overall efficiency of the PTR optical element. Additionally, the thermal systems utilized in an effort to control the temperature of a PTR optical element may be relatively bulky and, as a result, may be unable to be employed in a small laser cavity. For example, the thermal systems utilized in conjunction with a PTR optical element for purposes of thermal stabilization may include mounts that are required to house a relatively large thermoelectric cooler (TEC), relatively large cold plates requiring cooling water and corresponding electrical cords for supplying power. In certain applications of the laser system in which the laser system including the thermal control system must be placed in a vacuum chamber, the infrastructure to support the mounting of the thermal system may create challenges.